1. Field of the Invention
This application relates to UV and UV/moisture dual curable compositions with improved cure through volume, which may also demonstrate high resistance to flammability and combustibility.
2. Brief Description of Related Technology
Moisture-curing mechanisms, heat-curing mechanisms, and photo-initiated curing mechanisms are among the means used to initiate cure, i.e., cross-linking, of reactive compositions, such as reactive silicones. These mechanisms are based on either condensation reactions, whereby moisture hydrolyzes certain groups, or addition reactions that can be initiated by a form of energy, such as electromagnetic radiation or heat. In certain compositions, a combination of such cure mechanisms may be used to achieve the desired results.
For example, reactive polyorganosiloxanes can be cured by heat in the presence of a peroxide. Alternatively, these reactive siloxanes can also be cured by heat in the presence of silicone hydride-containing (—SiH) compounds and a metallic hydrosilylation catalyst, such as an organo-platinum catalyst.
UV curing silicones having methacrylate functional groups are known. For instance, U.S. Pat. No. 4,675,346 (Lin), the disclosure of which is hereby expressly incorporated herein by reference, is directed to UV curable silicone compositions including at least 50% of a specific type of silicone resin, at least 10% of a fumed silica filler and a photoinitiator, and cured compositions thereof.
Other known UV curing silicone compositions include those disclosed in Great Britain U.S. Pat. No. 1,323,869 [composition for plate-making in printing consisting of an organopolysiloxane containing a (meth)acrylate functional group, a photosensitizer, and a solvent, which cures to a hard film]; U.S. Pat. Nos. 4,201,808 (Culley) and 4,348,454 (Eckberg) (compositions of an organopolysiloxane having an average of at least one acryloxy and/or methacryloxy group per molecule; a low molecular weight polyacrylyl crosslinking agent; and a photosensitizer, reported as being suitable for release coatings and curable upon exposure to UV radiation); and Great Britain U.S. Pat. No. 2,039,287 (composition for protective coatings on paper prepared from the reaction of methacryloxypropyltrimethoxysilane and low molecular weight hydroxyl-terminated polyorganosiloxanes).
UV/moisture dual curing silicone compositions are also known. See U.S. Pat. Nos. 4,528,081 (Nakos) and 4,699,802 (Nakos), the disclosures of each of which are hereby incorporated herein by reference.
In certain commercial applications of such silicone formulations, flammability and combustibility is a concern, particularly where it is an issue in production or with the end user. As a result, ordinarily only a subset of such silicone formulations have been used in such instances.
In the past, to alleviate the concern, heat-curable silicone compositions have been used where moldability of the composition itself and electrical properties of the cured product were desirable. Such heat-curable silicone compositions have been rendered resistant to flammability and combustion through the addition of halogenated materials, fillers, organo-platinum materials, and benzotriazoles.
However, the use of halogenated materials to achieve that property is undesirable because of toxicity and environmental concerns, as well as possibly contaminating the substrate on which the composition is placed.
Other ways of rendering such heat-curable silicone compositions resistant to ignition are also known. For instance, European Patent Application Nos. 801 111, 808 874 and 808 875, each relate to liquid silicone rubber compositions which, after cure, are reported as producing a highly flame retardant silicone rubber. The compositions are polyorganosiloxane-based, and contain silica filler, aluminum hydroxide, benzotriazole, and a platinum-3,5-dimethyl-1-hexyn-3-ol reaction mixture. The compositions may further include zinc carbonate, polyorganohydrogensiloxane, and a hydrosilylation reaction catalyst.
In addition, a Chemical Abstracts citation to a Chinese patent document refers to a flame retardant silicone rubber for dielectric coatings of electric wire containing a vinyl-siloxane rubber, benzotriazole, chloroplatinic acid, a silica filler, and hydroxy silicone oil. See Chem. Abs. 128:168854s (1998)(citing Chinese Patent Document CN 1,144,237). Vinyl-siloxane rubbers are ordinarily curable by thermal mechanisms.
However, with certain commercial applications, heat-curable silicones present shortcomings. Those shortcomings include their limited usefulness with heat-sensitive substrates and/or heat-sensitive electronic circuitry. In addition, heat-curable silicones generally are slower to cure than silicones curable through other mechanisms, such as light cure. Such a shortcoming is seen to be troublesome in the fabrication of devices, where the slower cure speed renders the fabrication process otherwise less efficient. Therefore, the applicability of such heat-curable silicones across a wide range of end-use applications appears to be limited.
Indeed, in those applications where a UV curable silicone composition would be desirable, one could not simply use the flame retardant materials as they have been used in the past with heat-curable silicones to impart such properties. That is, certain of these filler materials generally (and in the amounts called for by the EP '111, '874 and '875 publications) would render the compositions less transmissive to light (and oftentimes opaque). Compositions containing such fillers, thus, may not be cured properly by exposure to UV radiation. That is, at best a surface skin may form, but cure through volume likely will not occur due to interference by the filler.
And, a Chemical Abstracts citation to a Japanese patent document refers to UV-curable polysiloxane coating compositions reportedly having flame retardant capabilities. These silicone compositions contain a siloxane component, 2-hydroxy-2-methyl-1-phenylpropane and aluminum acetylacetonate. These compositions do not however appear to be filled, and they therefore may not possess the strength required of a cured material in a sealant or conformal coating application.
Recently, Loctite Corporation has designed and developed UV and UV/moisture dual curable silicone compositions, which also demonstrate high resistance to flammability and combustibility, where the flame-retardant component is claimed to be the combination of hydrated alumina and a member selected from the group consisting of organo ligand complexes of transition metals, organosiloxane ligand complexes of transition metals, and combinations thereof. See U.S. Pat. Nos. 6,281,261 (Bennington) and 6,323,253 (Bennington)
A drawback to conventional radiation curable compositions is the degree of radiation penetration beneath the surface of the dispensed composition. Heretofore, the use of light transmissive fillers has been reported with certain curable compositions. See e.g. U.S. Pat. Nos. 5,362,769 (Wailer) (for instance at col. 4, lines 28-40), 4,309,334 (Valitsky) (for instance at col. 6, lines 53-63), 4,753,704 (Stewart) (for instance at col. 3, lines 6-14) and 4,252,526 (Madhavan).
Notwithstanding the state of the technology it would be desirable to provide a UV, and optionally moisture, curable composition, which possesses improved cure through volume properties and which demonstrates commercially acceptable tack free time upon curing. In addition, it would be desirable for such compositions to demonstrate flame- and combustion-retardant properties when cured.